A method of the kind mentioned in the opening paragraph is known from the European Patent Application no. EP-A 0 220 552, according to which TEOS vapour is conducted from the reservoir to the reactor chamber and the substance is heated to such a level that a vapour is generated in the reservoir having a comparatively high pressure and this vapour is then conducted through a resistance in the gas line to the reactor chamber which is kept at a constant pressure. If the vapour in front of this resistance has a pressure p.sub.i and the pressure in the process chamber is p.sub.p, the following holds for a laminar flow through the resistance R: EQU R=constant/(p.sub.i +p.sub.p)
A vapour flow Q.sub.d then moves to the reactor chamber for which it holds that: EQU Q.sub.d =(p.sub.i -p.sub.p)(p.sub.i +p.sub.p)/constant=(p.sub.i.sup.2 -p.sub.p.sup.2)/constant
The vapour flow Q.sub.d is kept at a desired, defined value in that the pressures p.sub.i and p.sub.p are maintained at desired, defined values.
In practice, the known method turns out to be not very flexible. If one wants to provide layers more quickly, for example, processes are taken with process pressures p.sub.p which are five to ten times higher than in the known method. If the same vapour flow Q.sub.d is to be maintained, the pressure p.sub.i must then be raised. Since the value of the resistance R can only be varied to a limited degree because of the type of flow in the gas line and the inertia of the vapour. It will also be necessary to increase p.sub.i further if the desire is to increase the vapour flow Q.sub.d which is conducted into the reactor chamber. In the known method, p.sub.i depends inter alia on the temperature of the reservoir. Thus, in the known method, p.sub.i is 307 Pa, while a pressure of 1067 Pa occurs in the reservoir at a temperature of approximately 45.degree. C. If much higher pressures p.sub.i are to be realised, the temperature of the reservoir must be raised. A drawback is, however, that this is practically impossible since the TEOS in the reservoir will be dissociated when the reservoir temperature exceeds approximately 80.degree. C.